Meet Mr Hilbert.....

German mathematician David Hilbert (1862-1943)
was the first to distinguish between mathematics
and metamathematics.

 He was also the first to discover the invariant theory.
At the Paris International Congress of Mathematicians
 in 1900, he presented 23 important questions that intrigued
 mathematicians over the century.

The founder of computers Alan Turing
was one of the mathematicians
who solved one of the 23 important questions,
thus paving the way for the 21st century
whist still in the first half of the 20th century
without which you would not be reading this.